(a) Technical Field
The present invention relates to an intelligent membrane electrode assembly (MEA) for fuel cell. More particularly, the present invention relates to an intelligent MEA for fuel cell that can easily measure the voltage of unit cells and prevent heat transfer from an outside low-temperature heat source to a catalyst layer of the MEA during preservation and operation of the fuel cell stack at low temperature.
(b) Background Art
A fuel cell stack, which is a main power source of a fuel cell vehicle, is a device that generates electricity by an electrochemical reaction between oxygen in the air and hydrogen supplied from fuel. The fuel cell stack applied to a vehicle is composed of several hundred unit cells, and each unit cell generates a voltage of about 0.6 V to 1.0 V.
FIG. 1 is a schematic diagram illustrating a conventional fuel cell stack composed of three unit cells.
As shown in the figure, the unit cell includes a membrane electrode assembly (MEA) 100, a gas diffusion layer (GDL) 102, a separator 103, and a gasket 101 for an airtight seal.
In general, the voltage of the unit cell is monitored through an electrical contact 104 formed on the separator 103 as a measurement terminal in order to check the state of the corresponding unit cell during the operation of the fuel cell stack.
Such a cell voltage monitor system (CVMS) includes a measurement terminal for forming the electrical contact 104 with the separator 103 and a control unit for measuring the voltage.
As mentioned above, since the fuel cell stack for vehicle is composed of several hundred unit cells, it takes a lot of time to form a voltage terminal in each unit cell after assembling the stack, which is considered as a problem that affects the production rate in mass production of the fuel cell stack.
According to a prior art technology, a plurality of grooves are is formed on the later surface of the separator 103, and a plurality of electrically conductive terminals acting as measurement terminals are sequentially inserted in the grooves. However, such a structure has some drawbacks in that the separators may be damaged by vibration and impact generated during vehicle driving as shown in the photographs of FIGS. 3 and 4. Moreover, the contact property with respect to the separator may be deteriorated and the terminals may be separated therefrom. Furthermore, it takes much time to sequentially insert the terminals into the grooves.
In order to overcome such drawbacks, U.S. Pat. No. 6,410,176 and U.S. Patent Application Publication No. 2003/0092292 disclose voltage monitoring systems in which an elestomeric connector is closely adhered to the lateral surface of a separator. Moreover, U.S. Patent Application Publication No. 2002/0090540 discloses an electrical contacting device for an electrochemical fuel cell, in which electrical contacts are formed on a printed circuit board (PCB) and the PCB is attached to the lateral surface of a separator. Besides, a method of monitoring the voltage using an electrical contact terminal to which an elastic force is added using a spring is known in the art.
However, the above-described prior art technologies have some common problems in that, as the interval of the separators is irregular, in case of a fuel cell stack composed of a small number of unit cells, it is possible to design a connector having elastic contact terminals arranged in series in view of manufacturing tolerance of the fuel cell stack and mount the thus designed connector to a separator; however, in case of a fuel cell stack composed of at least 200 unit cells, it is very difficult to design the contact terminals due to manufacturing tolerance of the whole length of the stack.
FIG. 2 is a schematic diagram illustrating a conventional MEA 100.
As shown in FIG. 2, an MEA 100 has a structure in which a catalyst layer 201 is coated on both sides of an ion exchange membrane 200 (or an electrolyte membrane) capable of transferring hydrogen protons so that hydrogen and oxygen react with each other, and a support film 202 is formed on both sides thereof for the reinforcement of the ion exchange membrane 200 and the convenience of handling.
The key problem to be solved to achieve the mass production of the fuel cell stack is startability at a low temperature below the freezing point.
Under low temperature conditions, if the ion conductivity of the ion exchange membrane 20 is sharply decreased, the performance of the fuel cell stack is deteriorated. Especially, vapor generated by a reaction between hydrogen and oxygen would be frozen on the catalyst layers 201 and thus the reaction would not occur.
Accordingly, starting the fuel cell at a temperature below the freezing point becomes a critical issue in many companies and research institutes related to the fuel cell.
In order to improve the startability at a low temperature of the fuel cell stack, the temperature of the fuel cell stack should be raised to a normal state within a short period of time, and the methods that have been proposed to improve startability include:
1) Installing an electrical heating device in the vicinity of a fuel cell stack connection device or an electrical current collector located on both ends of the fuel cell stack;
2) Wrapping the fuel cell stack with a heat insulating material to prevent heat generated in fuel cells from being transferred to the air and to use the heat to raise the temperature of the stack; and
3) Heating a coolant by electrical energy generated at the initial stage of the operation of the fuel cell stack and supplying the heated coolant to the fuel cell stack.
As described above, in order to start the fuel cell stack at a temperature below the freezing point to be operated in a normal state, the temperature of the stack should be raised to a temperature above the freezing point within a short period of time. However, the amount of energy generated in the fuel cell stack itself, while water, heat and electricity are generated by the reaction between hydrogen and oxygen, is not enough to raise the temperature of the stack above the freezing point.
Especially, the temperature of the fuel cell stack should reach the melting temperature before the reaction water is frozen on the surface of the catalyst layer and the electrochemical reaction thus cannot occur in the fuel cell stack. In order to realize the above-mentioned objective, U.S. Patent Application Publication No. 2006/0240300 discloses a combustion-thawed fuel cell structure in which combustion chambers are located adjacent to the end cells to conductively heat the end cells during cold start-up of a fuel cell stack. U.S. Patent Application Publication No. 2005/0277003 discloses a fuel cell system that employs a start-up heater coupled to a cold plate that warms a stack coolant during start-up of the system. Moreover, U.S. Pat. No. 6,916,566 discloses a system and method for rapid preheating of an automotive fuel cell, in which compressed air from an air supply compressor is used to heat fuel cells of a fuel cell stack using a heat exchanger.
However, although the above-described prior art methods heat end cells exposed to the cold air to raise the temperature thereof, they cannot raise the temperature of the entire fuel cell stack.
Moreover, the method of heating a coolant by electrical energy generated at the initial stage of the operation of the fuel cell stack or using a heat exchanger requires a lot of time to heat an intermediate material and is inefficient in view of the efficiency of the heat exchanger.
Furthermore, the method of wrapping the fuel cell stack with a heat insulating material is effective to prevent the temperature of the stack from falling after driving at a low temperature and to shorten the time required to raise the temperature of the stack above the freezing point by preventing heat generated in the stack from being transferred to the air; however, it cannot actively raise the temperature of the stack.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.